The invention relates to an internal combustion engine operating on spark-ignitible fuel, featuring at least one operating mode with homogeneous charge and self-ignition, and comprising at least one intake valve and at least one exhaust valve per cylinder, which intake and exhaust valves may be coupled via a transmission device in at least one operating mode of the engine.
By spark-ignitible fuel any conventional fuel of relatively high octane number is understood, such as gasoline.
The crucial determinants for the combustion process in an internal combustion engine are the timing of the combustion process or beginning of combustion, maximum rate of increase of the cylinder pressure, and peak pressure.
As regards an internal combustion engine in which combustion is essentially due to self-ignition of a homogeneous fuel-air mixture, these determinants are established by controlling charge composition and charge temperature curve. These two parameters in turn are determined by a large number of variables, such as engine speed, amount of fuel, intake temperature, charge pressure, effective compression ratio, inert gas content of the cylinder charge, and component temperature.
It has been found that charge temperature is of special importance in controlling the velocity of the chemical processes taking place during ignition delay and combustion proper. A most efficient means of raising charge temperature is an increase in the amount of residual gas, i.e., the content of non-scavenged exhaust gas from the previous combustion cycle remaining in the cylinder charge for the following cycle.
In AT 003 135 U1 an internal combustion engine is described, which in at least one operating mode burns a homogeneous fuel-air mixture by self-ignition of a fuel that is both spark-igniting and self-igniting, especially gasoline. In this way low-emission operation may be obtained at part-load. In order to control under part-load conditions the amount of residual gas internally recycled to increase charge temperature, a variable valve timing system is provided by which the closing time of at least one exhaust valve may be varied in dependence of engine operating parameters. Internal exhaust gas recirculation and control of the residual gas content in the combustion chamber will lead to a rise of the amount of inert gas, however. In order to be able to independently control the beginning of combustion and maximum combustion rate, it would be necessary to provide for control of the inert gas content independently of temperature requirements.
U.S. Pat. No. 5,546,914 presents a system for exhaust gas recirculation in a diesel engine, which includes an auxiliary element for activation of at least one of the intake valves during the exhaust stroke, thus permitting partial recirculation of the exhaust gas into the intake system. The valve timing device is conceived as a hydraulic unit and has a pumping piston and a working piston which are connected via a hydraulic chamber. The pumping piston is connected to a rocker arm of the exhaust valve and the working piston to a rocker arm of the intake valve, the pumping piston actuated by the exhaust rocker arm hydraulically acting on the working piston, which in turn acts on the intake valve in opening direction. By means of a solenoid valve forming the auxiliary element the hydraulic chamber may be hydraulically pressurized, resulting in an opening of the intake valve during the exhaust stroke. In this way an internal exhaust recirculation is obtained, where the exhaust gas is cooled down in the intake port until the inlet period sets in. The known system favors cooling of the exhaust gas in order to improve charging efficiency and nitrogen oxide emissions.
In low-temperature combustion systems used in engines operating on the HCCI (homogeneous charge compression ignition) principle, however, the internal exhaust gas recirculation serves the purpose of ignition timing by control of charge temperature, where high temperatures of the recycled exhaust gas are desirable.
U.S. Pat. No. 5,603,292 discloses a valve mechanism for an internal combustion engine comprising at least one intake valve and one exhaust valve. Intake and exhaust valves are driven by a camshaft via one rocker arm each. A mechanical transmission device permits opening of the exhaust valve at the same time as the intake valve moves towards its opening position. The transmission comprises a shaft with one pivot arm each in the area of intake and exhaust rocker arms. Via the shaft the rocker arm of the intake valve acts on the rocker arm of the exhaust valve in opening direction. In this way an internal exhaust gas recirculation may be accomplished even if the pressure ratio between exhaust and intake side is unfavorable. By an axial shift of the shaft the transmission device can be activated and deactivated by means of an hydraulic actuating device. Hydraulic actuation requires considerable design and control efforts, however.
It is the object of the present invention to permit improved control of the combustion process of low temperature combustion systems.
According to the invention this object is achieved by providing that at least one exhaust valve can be actuated during the intake stroke by means of the transmission device, said transmission device having at least one first mechanical transmission member, and that the transmission device be activated by a mechanical actuating device, said actuating device featuring an eccentric element acting on a transmission member. During this process a defined amount of hot exhaust gas is returned from the engine exhaust side into the cylinder, thus raising the charge temperature in the desired manner. At least one exhaust valve and one intake valve will be coupled and simultaneously operated during the intake stroke by the transmission device.
Activation of the transmission device is thus accomplished via a mechanical actuating device, which is preferably formed by an eccentric element acting on a transmission member. This results in a simple design for a low-cost control system.
According to a particularly simple embodiment of the invention the first transmission member is formed by a preferably two-armed tiltable first lever, whose first lever arm may be mechanically connected to intake actuation means of the intake valve and whose second lever arm may be mechanically connected to exhaust actuation means of the exhaust valve, in a direct or indirect manner, the lever actuated by the intake actuation means acting mechanically on the exhaust actuation means and activating them in the direction of the exhaust valve opening. If the camshaft is situated below the cylinder head the transmission member may directly cooperate with the intake actuation means and exhaust actuation means.
In the instance of an overhead camshaft, however, it will be better to provide for the intake actuation means to act indirectly on the first transmission member, via a second mechanical transmission member, which is preferably formed by a preferably two-armed tiltable second lever. As an alternative it may be provided that the first transmission member act indirectly on the exhaust actuation means via a second mechanical transmission member, which is preferably formed by a preferably two-armed tiltable second lever.
For operation of the transmission device the proposal is put forward that mechanical contact be established between the transmission member and the intake and/or exhaust actuation means via the eccentric element. In a particularly simple variant the eccentric element acts on the first lever in the area of the lever pivot axis, the first lever preferably turning about the lever pivot axis on the eccentric element.